1. Field of the Invention
The present invention relates to a brushless motor, which is small in rotational irregularity and low in vibration.
2. Description of the Related Art
FIG. 8 is a graph exhibiting an amount of deformation of a back yoke in a brushless motor according to the prior art for explaining a flatness of the back yoke.
FIG. 9 is a cross sectional view of the conventional brushless motor according to the prior art.
FIG. 10 is a plan view of the conventional brushless motor shown in FIG. 9.
FIG. 11 is a fragmentary view taken in the direction of the arrows substantially along the line M-M of FIG. 9.
FIG. 12 is a fragmentary view of the stator section of the other conventional brushless motor according to the other prior art.
Currently, brushless motors have been used in various electronic equipment. For instance, a brushless motor is used in a VTR (video tape recorder) as a motor for driving a tape and in a magnetic disc drive as a motor for driving a disc.
With respect to a prior art, such a brushless motor is disclosed in the Japanese publication of unexamined utility patent applications No. 6-2963/1994. The brushless motor disclosed in the Japanese publication of unexamined utility patent applications No. 6-2963/1994 is a capstan motor for driving a tape in a VTR, and constituted so as to be able to accurately achieve perpendicularity of a bearing holder and a rotary shaft with respect to a stator substrate.
More specifically, a joint surface of the bearing holder with respect to the stator substrate is approximately formed in a columnar shape with centering the rotary shaft, and resulting in enabling to achieve perpendicularity of the rotary shaft.
With referring to FIGS. 8-11, the conventional brushless motor 200, which is a capstan motor for being used in a camcorder, is described hereupon.
In FIG. 9, a motor base 210 that is molded from a material of resin is composed of a basement 211 and a pillar 215 that is provided upright on the basement 211. The basement 211 and a top end portion 216 of the pillar 215 are formed with a hole 211a and a hole 216a respectively, wherein each center axis of the holes 216a and 211a is common to each other. A ball bearing 222 is force fitted into the hole 211a of the basement 211.
Further, a ring pier 212, which protrudes downward on the bottom of the basement 211, is integrally formed around the hole 211a of the basement 211.
A hole of a back yoke 220 is fitted on the outer circumferential surface of the ring pier 212 and fixed thereto. The back yoke 220 is made from a magnetic material such as iron sheet, and provided with the hole almost in the middle. In this connection, an outer ring of the ball bearing 222 is positioned by the inner surface of the hole 211a and the back yoke 220 is positioned by the outer circumferential surface of the ring pier 212.
A sintered bearing 223 is force fitted into the hole 216a of the top end portion 216 of the motor base 210. Then the top end portion 216 is caulked, so that the sintered bearing 223 is fixed thereto firmly.
By the ball bearing 222 and the sintered bearing 223, a rotary shaft 225 made from stainless steel is rotatably supported.
On the bottom surface of the back yoke 220, as shown in FIG. 11, a plurality of driving coils 231, which are formed almost in a ring shape having a hollow section by winding a conducting wire around the hollow section, are provided around the rotary shaft 225. More accurately, six driving coils 231 are arranged at angular intervals of 60 degrees.
Further, as shown in FIG. 11, a hall element 235 is mounted in each hollow section of the three driving coils 231 that are adjacent to each other on the surface of the back yoke 220. The hall elements 235 are used for alternating electric power to be supplied to the driving coils 231.
Referring back to FIG. 9, description is given to a rotor section. The rotary shaft 225 protrudes through the ball bearing 222 downward, and a rotor section is mounted on a protruded portion 225a of the rotary shaft 225 so as to be integrated with the rotary shaft 225.
The rotor section is composed of a bushing 219, a rotor yoke 228, a gear 230, a driving magnet 224 and a plastic magnet 229 for frequency generator (hereinafter referred to as FG magnet 229).
More specifically, the bushing 219 that is made from brass is fixed on the protruded portion 225a of the rotary shaft 225, and the rotor yoke 228 is fixed around the bushing 219.
A gear 230 that is made from plastic is force fitted on a bottom end portion of the bushing 219. A timing belt not shown in the figure is extended around the gear 230, and drives a reel of a VTR.
The rotor yoke 228 is formed in a flat cup shape having a surrounding wall 228a. 
The driving magnet 224, which is made from a rare earth material and formed in a flat ring shape, is adhered on an inner surface of the surrounding wall 228a of the rotor yoke 224.
The driving magnet 228 is magnetized in eight poles in a circumferential rotating direction.
Further, a top surface of the driving magnet 224 confronts with a bottom surface of the driving coil 231 with maintaining a prescribed gap “g” between them.
An outer circumferential surface of the surrounding wall 228a of the rotor yoke 228 is integrally provided with the FG magnet 229 by a so-called outsert forming method (a projection forming method of engineering plastics).
By means of the FG magnet 229, an MR sensor 232, which is provided outside an array of the driving coils 231 as shown in FIG. 11, generates an FG signal.
In accordance with the FG signal, a rotational speed of the brushless motor 200 is adjusted to be a constant speed by controlling electric current supplied to each driving coil 231 through a flexible printed circuit board (herein after referred to as FPC) 213.
Generally, combinations of number of driving coils (Nc) and number of magnetic poles (Nm) to be selected are essentially as follows in case number of electric phases of a driving motor is three.
(Nc, Nm)=(3, 2), (3, 4), (6, 4), (6, 8), (9, 6), (9, 12), (12, 16), and so on.
In such a combination in which the number of magnetic poles (Nm) is 2n or 4n with respect to the number of driving coils (Nc) that is 3n, where “n” is a natural number, driving coils are evenly disposed in a circumferential area, with centering a rotary shaft.
Therefore, a belt for driving a reel could not be extended inside a magnetic circuit of the motor, and resulted in being obliged to be extended outside the magnetic circuit. Consequently, the motor was hardly made thinner in profile.
In this connection, a motor, which was intended to be made thinner in profile, has been proposed. The motor was provided with an angular area in which no driving coil was disposed with centering a rotary shaft.
Further, a hall element and a belt for driving a reel through a gear were disposed within the angular area.
Furthermore, the motor is configured such that combinations of number of driving coils (Nc) and number of magnetic poles (Nm) are defined as (Nc, Nm)=(3, 6), (6, 6), (6, 10), (9, 8), (9, 14), (12, 10), (12, 18), and so on.
In other words, the combinations are defined such that the number of magnetic poles (Nm) is “2n+2” or “4n+2” with respect to the number of driving coils (Nc) that is 3n, wherein “n” is a natural number, a not disposed angular area in which no coil is disposed within an angular range equivalent to two magnetic poles is provided, and a hole element and a belt are disposed therein.
Such a configuration of disposing driving coils is shown in FIG. 12.
As shown in FIG. 12, six driving coils 331 are disposed at angular intervals of 48 degrees, and an angular range from 135 degrees to 225 degrees approximately is provided as a not disposed angular area BS in which no driving coil is disposed.
Further, in FIG. 12, three hall elements 335 are disposed in the not disposed angular area BS.
By this configuration, an amount of magnetic flux interlinking with the driving coils 331 in total decreases, so that efficiency is deteriorated. However, there exists a merit of making the motor thinner in profile.
In the meanwhile, recently, market demands for a motor such as making smaller in size and improving performance higher are strong, so that component parts constituting a motor must be miniaturized and thinned in profile. However, miniaturizing and thinning decrease strength of component parts themselves.
In the above-mentioned conventional brushless capstan motor for camcorders, if the motor is thinned, a gap between the driving magnet 224 and the back yoke 220 or 320 is inevitably narrowed, and resulting in strengthening magnetic suction power. Consequently, the back yoke 220 or 320 is deformed furthermore.
In particular, as mentioned above, in case of the motor that is provided with the not disposed angular area BS in which no driving coil 331 is disposed around the rotary shaft 225 on the back yoke 320 through an FPC 313, the area in which the driving coils 331 are disposed is reinforced by adhering the driving coils 331 therein, and results in increasing flexural strength in comparison with the not disposed angular area BS in which no driving coil 331 is disposed.
As a result, the back yoke 320 is unevenly deformed toward the driving magnet 224 side by the magnetic suction power of the driving magnet 224.
In this connection, the gap “g” between the driving coils 331 and the driving magnet 224 is made uneven, and resulting in losing magnetic balance.
Accordingly, there exists problem such that wow and flutter (hereinafter referred to as W/F) and vibration increase.
Particularly, the back yoke 320 is defined as a base level of positioning for installation, in some cases, when installing a motor into a VTR or other electronic equipment. In this case, irregularity in flatness of the back yoke 320 directly reflects irregularity in perpendicularity of the rotary shaft 225. Consequently, affection of the irregularity in flatness of the back yoke 320 is extremely large.
An amount of deformation of the back yoke 320 in the longitudinal direction of the rotary shaft is shown in FIG. 8.
FIG. 8 is a result of measuring an amount of deformation at eight positions, which are disposed along a circle having a radius of approximately 11 mm from the rotary shaft at angular intervals of 45 degrees, wherein a position in the longitudinal direction of the rotary shaft at zero degree of the back yoke 320 having a radius of approximately 25 mm in FIG. 12 is defined as a reference height. The measurement is conducted before and after the rotor section is mounted in the stator section respectively.
Consequently, after the rotor section is mounted in the stator section, in other words, under a condition that the magnetic suction power of the driving magnet 224 acts on the back yoke 320, it is understood by FIG. 8 that deformation of the not disposed angular area BS in which no driving coil 331 is disposed is remarkably large. Particular, at an angle of around 180 degrees, the deformation reaches 150 μm maximal.
As mentioned above, providing a not disposed angular area in which no driving coil is disposed on a back yoke so as to thin a profile of a motor makes the back yoke deform unevenly due to magnetic action of a driving magnet, and resulting in making a gap between the back yoke and the driving motor uneven.
Accordingly, there exists the problem of increasing W/F and vibration.